The present invention relates to pre-expanded particles of a propylene resin and a process for preparing the same. More particularly, the present invention relates to pre-expanded particles of a propylene resin, which can be suitably used, for example, as a raw material for in-mold foamed articles, and a process for preparing the same. And the present invention relates to a flow-restricting device, which is used for a releasing part at preparing expanded particles by dispersing thermoplastic resin particles into an aqueous dispersion medium in a pressure vessel, impregnating a blowing agent and releasing the mixture into a lower atmosphere, and a drawing machine having the flow-restricting device.
There have been conventionally known a process for preparing pre-expanded particles of a propylene resin by dispersing particles of a propylene resin into an aqueous dispersion medium, and releasing the mixture into a lower atmosphere after heating and pressuring, and a process for preparing pre-expanded particles of a propylene resin by using a propylene resin containing a structure having two melting temperatures in a DSC curve measured by a differential scanning calorimeter in order to impart excellent in-mold moldability (disclosed in, for example, Japanese Unexamined Patent Publication No. 176336/1984).
A process for preparing pre-expanded particles of a propylene resin has been conventionally known, wherein an inorganic gas such as carbon dioxide gas, nitrogen or air is used as a blowing agent (disclosed in, for example, Japanese Unexamined Patent Publication Nos.221440/1985, 229936/1985, and 259724/1996).
However, if pre-expanded particles are prepared by using a propylene resin such as a random copolymer of ethylene and propylene and the above-mentioned inorganic gas as a blowing agent, it becomes impossible to expand the pre-expanded particles in a high expansion ratio. Since an expansion force of the blowing agent becomes insufficient due to a lack of solubility of an inorganic gas used as a blowing agent into a resin, or it becomes impossible to use an expansion force effectively as an expansion ratio of the pre-expanded particles due to a high transparency.
Therefore, the present inventors developed and filed an excellent method for preparing pre-expanded particles having desired properties by dispersing the propylene resin and resin particles, wherein a hydrophilic polymer is a base resin, into an aqueous dispersion medium, heating the mixture to at least a softening temperature of the propylene resin to obtain water-containing resin particles having a water content of 1 to 50%, and then releasing the mixture into a low atmosphere to expand the water-containing resin particles, by using water of a solvent as a blowing agent without a volatile blowing agent and/or an inorganic gas blowing agent (Japanese Patent Application No.84124/1996 specification). The method is an excellent process, since it is inflammable and safety, the blowing agent is cheap, it is gentle to environment and it becomes easy to increase an expansion ratio compared with the case in using an inorganic gas blowing agent such as air or nitrogen. But it has a drawback that it is difficult to increase the expansion ratio if a difference between the inside and outside pressure of the pressure vessel (expansion pressure) is not comparatively high, since it has a high boiling temperature and a high latent heat of vaporization compared with the conventional blowing agent.
The drawback is caused by the phenomenon wherein among the two melting temperatures shown in FIG. 1 an endothermic amount .DELTA.H at the higher melting temperature closely relates to an expansion ratio of pre-expanded particles, and the expansion ratio of the pre-expanded particles decreases linearly with increasing the endothermic amount .DELTA.H of the higher melting temperature. It is the same knowledge of the case in using an inorganic blowing agent such as carbon dioxide disclosed in Japanese Unexamined Patent Publication No.259724/1996.
On the other hand, it was reported that the endothermic amount of the higher melting temperature closely related to moldability for in-mold and it was necessary to maintain the endothermic amount in a constant range in order to keep excellent moldability for in-mold (for example, Japanese Unexamined Patent Publication No.20662/1996).
Therefore, it is necessary to maintain the endothermic amount in a constant range, in which moldability for in-mold is kept excellent, and to increase an expansion pressure in order to increase an expansion ratio.
There has been conventionally known a process for preparing pre-expanded particles of a propylene resin especially by using an inorganic blowing agent, which is a process for suitably preparing pre-expanded particles containing a crystalline structure having two melting temperatures in a DSC curve. For example, in Japanese Unexamined Patent Publication Nos.17615/1993 and 223347/1991, it is described that the objective product can be obtained by maintaining non-crosslinked polypropylene resin at around the expansion temperature usually for 5 to 90 minutes, preferably 15 to 90 minutes. But the above-mentioned maintaining is divided into two steps and schedule controlling is complicated. It has been understood that an effect of the expansion ratio is insufficient in the present invention by using two-step temperature controlling described in Example of Japanese Unexamined Patent Application No.17615/1993, wherein a difference between the maintaining temperature of the first step and the second step is 5 C, and a maintaining time at both the first step and the second step is 15 minutes.
On the other hand, Japanese Unexamined Patent Publication No.176336/1984 discloses that moldability for in-mold becomes excellent by increasing a temperature difference .DELTA.T of two melting temperatures of the pre-expanded particles of a propylene resin in a DSC curve measured by a differential scanning calorimeter. It is described that the temperature difference corresponding to .DELTA.T is preferably at least 5.degree. C. But in the publication there is no brief description of pre-expanded particles having .DELTA.T of above 13.degree. C. and a process for preparing the pre-expanded particles.
Conventionally, pre-expanded particles are generally prepared by heating particles of a thermoplastic resin dispersed into an aqueous dispersion medium in a pressure vessel, immersing a blowing agent, and releasing the mixture into a lower atmosphere to expand particles of a thermoplastic resin.
Releasing of particles of a thermoplastic resin is usually carried out by passing it through an opening provided at a flow-restricting device having an orifice type or nozzle type (disclosed in Japanese Unexamined Patent Publication No.197027/1983). The releasing is also carried out by passing it several times through an opening provided at a plurality of flow-restricting devices arranged in series toward releasing direction (disclosed in Japanese Unexamined Patent Publication No.4909/1988).
As a thermoplastic resin, examples are a styrene resin, a vinyl chloride resin, a polyamide resin, a polyester resin, a polyolefin resin and the like. Particles having a desired shape such as sphere, column, elliptical column or cube are prepared by using these resins. A volatile blowing agent, an inorganic gas or water is used as the blowing agent (disclosed in WO97/38048 pamphlet).
As a flow-restricting device of orifice type or nozzle type provided at the releasing part, there is generally used the plate having 1 to 10 slits, of which width is 4 to 20 mm, height is 4 to 20 mm, and depth at the narrowest part is 5 to 200 mm, or the plate having 1 to 10 holes, of which area is 10 to 400 mm.sup.2, and depth at the narrowest part is 5 to 200 mm. If an expansion ratio of expanded particles is increased by the above-mentioned method, it is carried out by increasing an inner pressure in a pressure vessel.
But if a volatile blowing agent is used as a blowing agent, it becomes necessary to increase an amount of the blowing agent and it becomes disadvantageous in production process and economy.
If expanded particles are prepared by using an inorganic gas or water as a blowing agent, it becomes disadvantageous in economy to obtain expanded particles having a desired high expansion ratio by increasing an inner pressure in a pressure vessel since the cost of the equipment becomes high.